[Enhancement]: "Go to other file" should create a body file for the current specification when not yet existing

[mgrojo]

Enhancement Request

After creating a specification which requires a body, it'd be good if pressing Alt+O (Ada: Go to other file) created a body skeleton for that specification. What the command is currently doing in that case is going to the parent package, which is unexpected by the user, and that operation deserves in fact its own command, so it can be invoked in any child package, regardless of whether a body file exists.

One possible implementation is to call gnatstub automatically.